In the past decade, the concept of using colloidal silica and bentonite to improve drainage, formation and retention has been introduced to papermaking. Fast drainage and greater retention of fines contribute to lower cost in papermaking and improvements are always being sought. U.S. Pat. Nos. 4,388,150 and 4,385,961 disclose the use of a two-component binder system comprising a cationic starch and an anionic, colloidal, silicic acid sol as a retention aid when combined with cellulose fibers in a stock from which is made. Finnish Published Specification Nos. 67,735 and 67,736 refer to cationic polymer retention agent compounds including cationic starch and polyacrylamide as useful in combination with an anionic silica to improve sizing. U.S. Pat. No. 4,798,653 discloses the use of cationic colloidal silica sol with an anionic copolymer of acrylic acid and acrylamide to render the paper stock resistant to destruction of its retention and dewatering properties by shear forces in the paper-making process. A coacervate binder, three component system composed of a cationic starch, an anionic high molecular weight polymer and dispersed silica having a particle diameter range from 1 to 50 nm is revealed in U.S. Pat. Nos. 4,643,801 and 4,750,974.
The above Finish publications also disclose the use of bentonite with cationic starch and polyacrylamides. U.S. Pat. No. 4,305,781 discloses a bentonite-type clay in combination with high molecular weight, substantially non-ionic polymers such as polyethylene oxides and polyacrylamide as a retention aid. Later, in U.S. Pat. No. 4,753,710, bentonite and a substantially linear, cationic polymer such as cationic acrylic polymers, polyethylene imine, polyamine epichlorohydrin, and diallyl dimethyl- ammonium chloride are claimed to give an improved combination of retention, drainage, drying and formation.
It is noted that the silica sol and bentonite are inorganic microparticle materials.
Latices of organic microparticles have been used in high concentrations of 30-70 lbs/ton to give "high-strength" paper products such as gasket materials, roofing felt, paperboard and floor felt and in paper with 30-70% mineral fillers (U.S. Pat. No. 4,445,970). It is stated that latices have not been used in fine papermaking because such latices are sticky and difficult to use on a Fourdrinier machine. The latices of the above and following four patent references were made according to U.S. Pat. No. 4,056,501. They are all emulsions of polymers made from styrene, butadiene and vinylbenzyl chloride which polymers are reacted with trimethylamine or dimethyl sulfide to produce an "onium" cation which is called a pH independent structured latex of 50 to 1000 nm in diameter. These structured cationic latices are used at high levels of concentration i.e. 30-200 lbs/ton either alone (U.S. Pat. No. 4,178,205) or with an anionic, high molecular weight polymer, (U.S. Pat. No. 4,187,142) or with an anionic polymer (U.S. Pat. No. 4,189,345) or as both cationic and anionic latices (U.S. Pat. No. 4,225,383). These latices are preferably from 60-300 nm in size It has been found, in accordance with the present invention, that noncrosslinked organic microbeads of this size and larger are not effective. Furthermore, the process of the present invention uses organic microbeads at a level of 0.05 to 20 lbs/ton, preferably 0.10 to 7.5 lbs/ton whereas the microbeads of the proceeding five U.S. Patent are used at 30-200 lbs/ton to give strength to paper products such as gaskets with a very high 30-70% mineral content. This prior art does not contemplate the use of charged organic micro-beads as a drainage and retention aid at the very low levels as required by the present invention.
The use of an organic crosslinked microbead, in papermaking is taught in Japanese Patent Tokkai JP235596/63:1988 and Kami Pulp Gijitsu Times, pgs 1-5, March 1989 as a dual system of a cationic or anionic organic microbead of 1-100 microns and an anionic, cationic or nonionic acrylamide polymer. The waterswelling type, cationic, polymer particle is a crosslinked homopolymer of 2-methacryloyloxyethyl trimethylammonium chloride or a crosslinked copolymer of 2-methacryloyloxy-ethyl trimethylammonium chloride/acrylamide (60/40 weight percent). The acrylamide polymer is an acrylamide homopolymer or acrylamide hydroylsate of 17 mole percent anion-conversion or a copolymer of acrylamide/2-methacryloyloxyethyl trimethylammoniumchloride (75/25 weight percent). The anionic microbead is an acrylamide-acrylic acid copolymer.
EPO 0273605 teaches the addition of microbeads having a diameter ranging from about 49-87 nm and produced from terpolymers of vinyl acetate (84.6), ethyl acrylate (65.4) and acrylic acid (4.5) or methacrylonitrile (85), butyl acrylate (65) and acrylic acid (3). These polymeric beads are disclosed as added to an LBKP pulp slurry in order to evaluate the resultant paper for sizing degree, paper force enhancement and disintegratability. These polymer beads fall outside the scope of those used in the present invention in that the ionic content thereof is too small to impart any appreciable improvement in retention and drainage in the papermaking process.
The present invention encompasses crosslinked, ionic, organic, polymeric microbeads of less than about 750 nm in diameter or microbeads of less than about 60 nm in diameter if noncrosslinked and water-insoluble, as a retention and drainage aid, their use in papermaking processes, and compositions thereof with high molecular weight polymers and/or polysaccharides.
EP 0,202,780 describes the preparation of crosslinked, cationic, polyacrylamide beads by conventional inverse emulsion polymerization techniques. Crosslinking is accomplished by the incorporation of difunctional monomer, such as methylenebisacrylamide, into the polymer chain. This crosslinking technology is well known in the art. The patent teaches that the crosslinked beads are useful as flocculants but are more highly efficient after having been subjected to unusual levels of shearing action in order to render them water-soluble.
Typically, the particle size of polymers prepared by conventional, inverse, water-in-oil, emulsion, polymerization processes are limited to the range of 1-5 microns, since no particular advantage in reducing the particle size has hitherto been apparent. The particle size which is achievable in inverse emulsions is determined by the concentration and activity of the surfactant(s) employed and these are customarily chosen on the basis of emulsion stability and economic factors.
The present invention is directed to the use, in papermaking, of cationic and anionic, crosslinked, polymeric, microbeads. Microgels are made by standard techniques and microlatices are purchased commercially. The polymer microbeads are also prepared by the optimal use of a variety of high activity- surfactant or surfactant mixtures to achieve submicron size. The type and concentration of surfactant should be chosen to yield a particle size of less than about 750 nm in diameter and more preferably less than about 300 nm in diameter.